SUNNYVALE -- Another round of fogging is set for the Silicon Valley next week, this time in Sunnyvale, Los Altos and Mountain View, the Santa Clara County Vector Control District announced Wednesday.

The fogging treatment, meant to help rid areas of mosquitoes that have tested positive for West Nile virus, is the third scheduled fogging in the past week.

The district said the mosquitoes in Sunnyvale, Los Altos and Mountain View tested positive for the virus on Tuesday. A fogging treatment is set for next Tuesday, starting at 11 p.m.

The fogging area borders Lynn Way, Cambridge Avenue, Brookline Drive and Hyde Park Drive on the north; Tilton and Yukon drives on the east; West Homestead Road on the south; and Alford, Park Hills and Wessex avenues on the west.

Mosquitoes in San Jose, Campbell and Los Gatos tested positive for West Nile virus last week. One fogging treatment in San Jose and Campbell was conducted Tuesday. Another is set for those same cities along with Los Gatos on Thursday.

County mosquito abatement staffers said that this year's mosquito season has already broken the record for early incidents of West Nile detection, and 2013 was the third most active year recorded.

More than a dozen residents voiced concerns about fogging at a June 10 Santa Clara County Board of Supervisors meeting, some calling the chemicals more harmful than West Nile virus and others wondering about its effectiveness.

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While vector control staff said studies of the same chemicals air-dropped in the Sacramento area showed no averse health effects and a sixfold reduction in the chance of contracting the virus, supervisors said they would like to see the concerns addressed and asked staff to return with information at a committee meeting after next month's recess.

An environmental report is due out in July, according to vector officials, which will be condensed in a staff report to focus on key questions about safety and effectiveness, as well as possible alternatives.

"The sooner we can either put concerns to rest or start thinking, 'OK, how might we do this different going forward,' the better," Supervisor Joe Simitian said. "Leaving these questions lingering longer than necessary is in no one's best interest."